The present invention relates generally to semiconductor manufacturing processes, and, more particularly, to a method for controlling voiding and bridging in silicide formation.
In the manufacture of semiconductor devices, salicide (or self-aligned silicide) materials are formed upon gate conductors and diffusion regions to reduce the line resistance of a CMOS device, thereby improving the speed characteristics thereof. In salicide technology, a refractory metal or a near noble metal, such as titanium for example, is deposited on a silicon substrate. The deposited titanium is then annealed, thereby forming a silicide layer only on the exposed areas of the substrate. The areas of unreacted titanium left on the dielectric may then be selectively etched away without a masking step. Thus, the process is “self-aligning.”
As circuit devices have continued to shrink in size, however, it has been found that titanium silicide (TiSi2) becomes an unsatisfactory silicide material since the sheet resistance thereof begins to sharply increase when the linewidth of the device decreases below 0.20 μm. More recently, cobalt disilicide (CoSi2) has been used as a replacement for titanium in salicide structures since it does not suffer from a linewidth dependent sheet resistance problem. On the other hand, the use of cobalt silicide structures is not without its own drawbacks. For example, unlike titanium, a cobalt layer requires a cap layer such as titanium nitride (TiN) due to the sensitivity of cobalt to contaminants during the annealing process.
When cobalt reacts with silicon on the active areas of a device, there is a dimensional reduction of the material from that of the original cobalt metal-free surface due to material densification during silicide formation. In other words, if one (volumetric) unit of metal is deposited on the top and sides of a gate structure, for example, after the silicidation process, the overall height and width of the structure will shrink depending on the extent of reaction on specific planes of the structure. These dimensional changes for different parts of the device structure require that the protective cap deposited over the silicide metal either flex due to changes in different areas of the device below, or allow voids to open up in certain locations thereunder to compensate for volumetric changes occurring at various points below the cap. Unfortunately, the differential stresses, voiding and/or delamination that can occur could allow silicon and/or metal to move into undesired areas, resulting in subsequent voiding and electrical bridging.